The conventional fermentation for the production of an ethanol containing solution must operate within the conditions required for cultivating the ethanol producing microorganism, e.g., Saccharomyces cerevisiae including, for example, maintenance of pH between about 3 and 7, maintenance of a temperature range between about 25.degree. C. and 38.degree. C., commencement of the fermentation with a wort containing not more than about 20% glucose by weight and avoidance of any efforts to generate more than about 10% by weight of alcohol in the fermentation broth.
In the great many instances wherein starch or a starchy substance such as corn grits is the source of the glucose consumed in the fermentation, the process includes a starch liquefaction and hydrolysis sequence to convert the (solid) starch into a glucose solution that becomes, in part at least, the growth medium for the yeast.
While starch conversion into glucose and maltose for fermentation purposes has been carried out on a large scale for eons in commercial practice, the procedures employed in prior art practices rarely convert starch completely into fermentable sugars. Even relatively minor deficiencies in conversion of starch into the fermentables, maltose and glucose, have adverse affect on the fermentation, the least of which is lower yield than would otherwise result, and/or a greater processing expense. For example, the traditional process for making beer wherein grains are hydrolyzed by malt results in a wort with a significant nonfermentable polysaccharide content, and, in turn, a beer with a significant polysaccharide content.
This situation has, of course, received considerable attention from workers in the pertinent arts.
On the whole, it is fair to state that starch liquefaction and hydrolysis procedures capable of producing a pure glucose syrup are available, as for example, liquefaction of starch according to the procedures described in U.S. Pat. No. 3,912,590 followed by saccharification according to the procedures described in U.S. Pat. No. 4,017,363. However, all of the starch liquefaction and hydrolysis procedures known to the inventor hereof can be criticized for requiring moderate to large quantities of thermal energy.
It should be noted moreover that fermentation of starch derived glucose syrups, whether of high purity or otherwise, face the microorganism with the presence of a very large excess of the glucose nutrient at the onset of the fermentation, little nutrient at the termination of fermentation, and otherwise comply with a need to avoid commencement of the fermentation with a syrup containing more than about 20% dissolved carbohydrate.
Insofar as the inventor herein is aware, the existence of disadvantages in fermenting a completely saccharified starch have received relatively little attention from the art. Instead the art has concerned itself with improving liquefaction, e.g., U.S. Pat. No. 3,912,590, saccharification to pure glucose, e.g., U.S. Pat. No. 4,017,363, resolving processing difficulties, e.g., U.S. Pat. Nos. 3,922,196 to 3,922,201 and 4,009,074; and with avoiding high thermal energy requirements for starch liquefaction and saccharification, e.g., U.S. Pat. No. 4,092,434. Some of the above referenced U.S. Pat. Nos., particularly 4,009,074 and 4,092,434, teach that ungelatinized starch, i.e., granular starch can be liquefied enzymatically at relatively low temperatures.
However, the inventor herein is not aware of any efforts by the art to integrate low temperature enzymatic liquefaction of starch with conduct of the fermentation so as to achieve improvements in the fermentation process, e.g., improved fermentation efficiency, and/or reduced fermentation time, and/or fermentation with a fermentation broth containing more than about 25% solids content. The last reduces water and energy costs.
The rationale of the present invention derives from a discovery that the fermentation may be carried out on a slurry of solid and completely ungelled starch, i.e., granular starch, dosed with alpha-amylase and glucoamylase. During the course of the fermentation the starch is enzymatically liquefied and saccharified into fermentable sugars and the sugars are fermented. Control over the fermentation rate is possible through variations in the starch concentration in the slurry, by preconditioning of the starch, and through variations in the concentration and proportions of alpha amylase and glucoamylase in the slurry.